1. Field of the Invention
The invention relates to superconductive circuits, and in particular, to semiconductive substrates rendered highly conductive by degenerate doping.
2. Description of the Prior Art
Superconductive memory and logic circuits are known in the art which utilize tunnel junctions for the active switching elements or gates including Josephson devices and QUITERON. It is important for reliable and reproducible fabrication of superconductive integrated circuits to maintain precise control of the values of normal resistors used in the circuits. Normal resistors are herein defined as resistive elements which maintain substantially constant resistance values whether the gate is in the zero-resistance or non-zero resistance state when operated at superconducting temperatures. Such normal resistors have heretofore been fabricated by depositing a thin metallized resistive film on an insulating substrate. These films are then etched and patterned to form the desired resistive network. Methods for rendering organic polymers conductive by ion implantation have also been discussed. While high conductivities are achieved, such polymers are unsuitable for use at superconductive temperatures. Implantation of ions to modify the conductivity of inorganic insulators, such as glasses and metal oxides, has also been taught. Ion implantation techniques have also been utilized to form resistive structures on a semiconductor wafer by bombarding the wafer surface with high energy ions. However, these techniques have not heretofore been extended to superconductive integrated circuits. A method for forming a normal resistor by altering a tunnel junction itself has been described by Lawrence N. Smith in Ser. No. 468,603, Superconductive Tunnelling Junction Resistor and Method of Fabrication, filed Feb. 22, 1983, assigned to the assignee of the present invention. That invention comprises a superconductive junction of superposed layers of superconductive material with a barrier layer therebetween, wherein at least one of the superconductive layers has been treated so as to remain in the normal state when the junction is operated at superconducting temperatures, whereby providing a non-zero resistance. However, this approach does not provide for free selection of the resistance value. Another approach has been described by H. Kroger in Ser. No. 468,604, Fabrication of Superconductive Tunneling Junction Resistors and Short Circuits by Ion Implantation, filed Feb. 22, 1983, and also assigned to the assignee of the present invention. This method permits transforming a Josephson junction into a normal resistor or even a short circuit. However, it requires active disruption of the barrier or "poisoning" of the superconductive electrodes. While it offers great flexibility in providing a wide range of resistance values, the fact that the barrier may be disrupted during the processing requires close control to avoid disrupting other Josephson junctions in proximity.
The present invention overcomes these disadvantages of the prior art by using a semiconductor substrate to form resistive elements. Such a resistor may be formed by the bulk resistivity of a diffused region in a semiconductive substrate. This is accomplished by introducing a dopant into the substrate by a diffusion process. Silicon wafers, for example, are commonly used as the substrates for Josephson logic fabrication. In the semiconductor art, silicon and germanium deposition technology has been developed to a high degree over the past 10-15 years so that in this area results having commercial uniformity are routinely achieved. Of all the semiconductors, silicon and germanium have been extensively studied and the properties and behavior are thoroughly understood. Further, polished single-crystal silicon wafers are readily available at relatively low cost as a result of their widespread use in the semiconductor industry, and silicon has a beneficially high thermal conductivity at 4.degree. K., a desirable feature for the operation of a computer using Josephson integrated circuits. But before the present invention, this silicon served no electronic function, merely affording a passive convenient mechanical substrate with good thermal properties. It is known that these semiconductor materials can be precisely and readily doped to provide either n or p type material with a precisely controlled Fermi level from intrinsic to degenerate. This invention makes use of the electronic conduction possible in degenerately doped silicon at 4.degree. K. to incorporate normal resistive elements as an integral part of the superconductive circuits.